Volumetric determination of perchlorates in polysulfide propellants



United States Patent 3,207,584 VGLUMETRIC DETERMINATIGN 0F PERCHLO- RATES IN POLYSULFIDE PROPELLANTS Bernard J. Alley and Hiram W. H. Dykes, Huntsville,

Ala., assignors to the United States of America as represented by the Secretary of the Army No Drawing. Filed Mar. 30, "1964, Ser. No. 355,969 Claims. (Cl. 23-230) (Granted under Title 35, US. Code (1952), sec. 266) This invention may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to an improved method for making determinations of perchlorate content in perchlorate containing material. Particularly, the invention concerns an improved volumetric method for determining the amount of perchlorate present in combustible compositions wherein the perchlorate compounds serve as oxidizers.

As is well known, there is a direct correlation between the performance of a given combustible composition and the oxidizer content of that composition. Thus, the ignition capabilities of pyrotechnic igniters and the ballistic performance of missiles which are powered by solidfueled rocket engines are directly related to the oxidizer concentration of the igniter composition and fuel composition, respectively.

The perchlorates comprise one of the major classes of oxidizers utilized in pyrotechnic and propellant compositions. Of the perchlorates, the alkali metal perchlorates and ammonium perchlorate are frequently employed with potassium perchlorate and ammonium perchlorate being used most often. Because of the direct relation of the performance of pyrotechnic and propellant compositions with the oxidizer concentration, it is essential that a satisfactory method of analysis be available to determine the amount of perchlorate present in any given sample of perchlorate-containing composition.

At present, two classes of volumetric procedures are employed to analyze pyrotechnic and propellant compositions for perchlorate content. The first is a wet chemical method involving reduction of the perchlorate ion followed by back titration of the excess reducing agent. In this method titanous reducing agents are generally used to reduce the perchlorate ion and then the excess titanous reducing agent is determined by back titration. However, in analyzing pyrotechnic compositions, the reducing and oxidizing agents present in the composition seriously interfere with the back titration of the titanous ion. Moreover, the titanous reducing agent must constantly be maintained in a non-oxidizing atmosphere or it will produce erroneous results in analytical procedures. The second method usually involves thermal reduction of the perchlorate ion and titration of the chloride formed. Thermal reduction of the perchlorate ion is time-consuming and requires special apparatus and safety precautions.

The method of the present invention is a wet chemical method which utilizes titanium hydride as the perchlorate reducing agent. Titanium hydride rapidly reduces the perchlorate ion and no special storing or handling of the reducing agent is necessary. The amount of perchlorate present is determined by titrating the chloride ion formed with standard aqueous solutions of silver nitrate. Knowing the chloride ion content, it is a routine calculation to determine the amount of perchlorate necessary to furnish the amount of chloride ion titrated. The present method is sufliciently rapid, precise, and accurate for use both in the assay of perchlorate compounds and the routine control determination of perchlorates in finished pyrotechnic and propellant compositions. Furthermore the "Ice process permits potentiometric titration of the chloride ion if desired.

In the present method, a carefully weighed sample of the perchlorate-containing composition is placed in a suitable container. Then an amount of titanium hydride in excess of the stoichiometric amount required to reduce the perchlorate ions present to chloride ions is added to the container. Thereafter aqueous sulfuric acid is added to the container and the entire contents are thoroughly mixed. The application of heat or, preferably, refluxing insures complete reduction of the perchlorate ions. Since the tripositive titanium ion interfers in the titration of the chloride ion, excess ammonium persulfate is subsequently added to the aqueous sulfuric acid solution containing the chloride ions to oxidize all the titanium ions to the tetrapositive state. Of course, those titanium ions involved in the reduction of the perchlorate ion are already in the tetrapositive state. After the ammonium persulfate dissolves and all tripositive titanium ions have been oxidized to the tetrapositive state as evidenced by the decolorization of the solution (the tripositive titanium ion imparts a violet color to the solution which disappears upon oxidation to the tetrapositive state), the chloride ion is titrated with a standard aqueous silver nitrate solution.

After the perchlorate ion has been reduced, the solution can be allowed to stand for an extended period of time without loss in accuracy. However, for reasons to be discussed later, once the ammonium persulfate is added to the solution, the entire method should be completed as rapidly as possible.

Since the present method is directed to the assay of perchlorate compounds and the determination of perchlorate content in pyrotechnic and propellant compositions, the approximate perchlorate content of an unknown sample will generally be available. Therefore, it is a matter of routine stoichiometry to determine the amount of titanium hydride required to reduce the perchlorate present in the sample. To insure complete reduction of the perchlorate, at least a slight stoichiometric excess of the titanium is used. If the approximate perchlorate content is known With a reasonable degree of accuracy, for example i5%, there is little, if any, advantage in using titanium hydride in excess of of the stoichiometric amount. In the event that the approximate perchlorate content of a given sample is not known, it is desirable to run a preliminary analysis to ascertain this approximate content.

For very precise determinations such as an assay of a reagent grade perchlorate compound, corrections must be made for such impurities as free chlorides and chlorates which would result in erroneously high perchlorate determinations. The concentrations of these impurities are generally very small and are regularly determined as a part of the initial perchlorate acceptance testing. For routine control it is suflicient to analyze for the perchlorate content, both before and after its incorporation in the pyrotechnic and propellant mixtures, without correction for these impurities.

Knowing the approximate amount of perchlorate to be reduced and the approximate excess of titanium hydride employed, it is again a matter of routine stoichiometry to determine the amount of ammonium persulfate required to oxidize the excess titanium ions to the tetrapositive state. Again, it is desirable to use a slight excess of ammonium persulfate, up to 150% of the stoichiometric amount. However, since a distinct decolorization accompanies the oxidation of the titanous ion to the tetrapositive state, it is possible to add ammonium persulfate in increments until decolorization occurs and then add a small additional quantity as an excess.

It should be pointed out that ammonium persulfate oxidizes the chloride ion under the conditions of the process. However, the oxidation occurs so slowly that no error is detected in the analysis if the chloride ion is titrated within fifteen minutes after adding the ammonium persulfate.

It has been determined that the sulfuric acid concentration is somewhat critical in obtaining very accurate determinations. Aqueous sulfuric acid containing from about 33% by weight to about 43% by weight dihydrogen sulfate is preferred with an acid having a dihydrogen sulfate content of about 38% being most preferred. This latter concentration is easily achieved by adding one volume of 98% concentrated sulfuric acid to three volumes of water.

One advantage to the sulfuric acid is that it breaks down the polymeric binders in some solid propellants. For example, the polysulfide-perchlorate propellants shown in US. Patent No. 2,997,376 can readily be analyzed by the present method because the sulfuric acid breaks down the cured polysulfide polymer and, therefore, frees" the perchlorate for analysis. If the cured binder is not affected by the sulfuric acid, an accurate analysis cannot be made since some of the perchlorate will be protected by the polymer and not reduced. In any event the method can still be used in analyzing samples prior to curing of the polymeric binder.

Another factor which contributes to accuracy, particularly if the chloride ions are titrated potentiometrically, is the removal from the aqueous sulfuric acid solution of substantially all insoluble solid material after reduction of the perchlorate ion and before adding the ammonium persulfate. This decreases electronic noise and prevents possible fouling of the electrodes. Ordinary filtration techniques accomplishes the removal of the solids satisfactorily.

The weight of sample selected for treatment and potentiometric titration should preferably provide from about 0.17 gram to about 0.22 gram of perchlorate ion for maximum accuracy. Larger amounts as well as smaller amounts can be analyzed with a possible sacrifice in overall accuracy.

The following example illustrates the present process for determining perchlorate concentration.

Transfer a sample containing 0.20 to 0.25 gram of ammonium perchlorate or equivalent amount of other perchlorates (weighed to i0.1 mg.) to a 250 milliliter widemouthed Erlenmeyer flask having a ground glass joint. Add to the flask, in this order, 0.5 gram of titanium hydride, 30 milliliters of Water, and milliliters of concentrated sulfuric acid (98%, specific gravity 1.84). Connect the flask to a water-cooled Liebig condenser having a 30- centimeter jacket and reflux the solution at a moderate rate on an electric heater. Remove the condenser and flask from the heater, rinse down the insides of the condenser and flask neck with water, and place the flask in a Water-ice bath. Cool the solution to C. (to prevent loss of hydrogen chloride gas) and filter it through a 40-milliliter Gooch crucible, fitted with an asbestos mat, into a ZOO-milliliter Griifin beaker. Just prior to titration, add 2.0 grams of ammonium persulfate to the solution and stir until decolorization occurs. Thereafter titrate the chloride ion with a standard aqueous silver nitrate solution and calculate the percentage of perchlorate present in the conventional manner.

A few trials may be required to establish a suitable reflux rate during reduction of the perchlorate ion. If the initial rate is too rapid, excessive foaming occurs and hydrogen chloride can possibly escape through the condenser, particularly if the condenser and flask walls are dry. If the reflux rate is too slow, the perchlorate ion may not be reduced during the refluxing operation. Using the equipment and procedure set forth above, an equilibrium reflux rate of twenty drops per minute insured reduction of the perchlorate ion in fifteen minutes. Since this particular procedure involves the step of adding concentrated sulfuric acid to water, the sulfuric acid solution is already hot at that pointa factor which expedites the overall method.

The samples discussed hereinafter were potentiometrically titrated with 0.1 N silver nitrate reagent using a Sargent-Malmstadt spectro-electrometric titrator (Sargent No. S29700). The electrode combination consisted of a shielded platinum reference electrode immersed in the silver nitrate reagent, and a platinum ring indicator electrode surrounding the glass stirrer. The titrant flow rate was maintained at 6 milliliters per minute by a 50 milliliter controlled delivery buret. The grid bias on the first thyratron tube in the control unit was set at the factoryrecommended value of 4.0 volts DC. with respect to ground and the voltage was checked periodically.

All instrumental operating conditions are preferably adjusted to provide a maximum potential change at the titration end point with a minimum of electronic noise. The magnitude of the potential change at the end point of the chloride titration depends primarily on the concentra tions of chloride and titrant, the titrant flow rate, and the rate of stirring during titration. All these factors should have the maximum possible value for best results. The six milliliter per minute titrant flow rate is the maximum recommended by the titrator manufacturer.

Because of the high titrant flow rate, calculations were made of the linear regression of milliliters of silver nitrate solution on grams of a sodium chloride standard. The data was statistically evaluated and the null hypothesis that the intercept was equal to zero was tested and accepted at the confidence level. This is consistent with the mean errors of the data presented in Tables I, II, and IV.

Using the method outlined above various perchlorate determinaitons were made. The results of these determinations are tabulated in Tables I-IV below. The titanium hydride powder was a commercial item (]17l5 AR purchased from Metal Hydrides, Incorporated, Beverly, Massachusetts). The water was distilled and essentially free of chlorides. With the exception of the perchlorates, the materials which comprised the synthetic mixtures were the same polytechnic grades used in actual production. The remaining ingredients used in the procedure were reagent grade.

The optimum weight-range of perchlorate was determined by analyzing reagent-grade potassium perchlorate. The results of analyzing ten successive samples are shown in Table I. The small estimated standard deviation and the mean error of zero establish the worth of the method in the assay of potassium perchlorate. Comparable results were achieved with ammonium perchlorate analysis.

TABLE I Assay of reagent-grade potassium perchlorate Table II gives the perchlorate determination results from the analysis of intimate admixtures of various igniter compositions.

TABLE H Analysis of igniter mixtures Ingredients, weight Percent Percent Weight of sample, grams K0104 Al Ti B Nylon Poly'iso Potassium Found 1 butylene Perchlorate Average 100. 00 Standard Devi i n 0.

1 This column presents the ratio (expressed as a percentage) of the weight of potassium perchlorate found by the method of the invention to the weight of potassium perchlorate actually used in preparing the composition.

Table III illustrates the reproducibility of perchlorate determinations made by the method of the present invention. Duplicate samples of various igniter compositions were run and the results compared.

TABLE III Reproducibility of determinations The perchlorate analyses of several cured polysulfideammonium perchlorate propellant samples (of known composition) of the type shown in US. Patent 2,997,376 are tabulated in Table IV.

TABLE IV Polysulfide-perchlorate propellant analyses Actual Weight of Percent Sample No. NH4C104 in NH C1O Sample, Found 1 Grams Avera e 100. 00 Standard Deviation 0. 13

l See definition Table II.

Many obvious modifications to the invention will be immediately apparent to those skilled in the art. Therefore, no limitation to the scope of the invention is intended by the above detailed description thereof except as reflected in the appended claims.

We claim:

1. The method of determining the amount of perchlorate ion present in a sample of perchlorate-containing material, said method comprising the steps of:

(a) Reducing the perchlorate ion with excess titanium hydride in the presence of aqueous sulfuric acid;

(b) Oxidizing the excess tripositive titanium ion with excess ammonium persulfate; and

(c) Thereafter titrating the chloride ion with a standard aqueous solution of silver nitrate.

2. The method of determining the amount of perchlorate ion present in a sample of perchlorate containing material, said method comprising the steps of:

(a) Reducing the perchlorate ion with excess titanium hydride in the presence of aqueous sulfuric acid, said acid containing from about 33% to about 43% be weight hydrogen sulfate;

(b) Oxidizing the excess tripositive titanium ion with excess ammonium persulfate; and

(c) Thereafter titrating the chloride ion with a standard aqueous solution of silver nitrate.

3. The method according to claim 2 wherein said aqueous sulfuric acid contains about 38% by weight hydrogen sulfate.

4. The method of determining the amount of perchlorate ion present in a sample of perchlorate-containing material, said method comprising the steps of:

(a) Reducing the perchlorate ion with excess titanium hydride in the presence of aqueous sulfuric acid, said acid containing from about 33% to about 43% by weight hydrogen sulfate, and thereafter removing substantially all undissolved solids from the resulting solution;

(b) Adding excess ammonium persulfate to substantially the solid-free solution thus formed to oxidize the tripositive titanium ions; and

(c) Thereafter titrating the chloride ion with a standard aqueous solution of silver nitrate.

5. The method according to claim 4 wherein said aqueous sulfuric acid contains about 38% by weight hydrogen sulfate.

6. In a solid combustible mixture comprising essentially fuel particles and a perchlorate oxidizer for said particles, the method of determining the amount of perchlorate oxidizer present in a sample of said mixture said method comprising the steps of:

(a) Contacting excess titanium hydride with said sample in the presence of aqueous sulfuric acid and thereafter removing substantially all undissolved solids from the resulting solution;

(b) Adding excess ammonium persulfate to the substantially solid-free solution thus formed to oxidize the tripositive titanium ions; and

(c) Titrating the chloride ion with a standard aqueous solution of silver nitrate.

7. The method according to claim 5 wherein said aqueous sulfuric acid contains from about 33% to about 43% by weight hydrogen sulfate.

8. The method according to claim 7 wherein said aqueous sulfuric acid contains about 38% by weight hydrogen sulfate.

9. In solid propellant compositions consisting essentially of an intimate admixture of fuel particles, a perchlorate oxidizer therefor, and a polysulfide hinder, the

method of determining the amount of perchlorate oxidizer present in a sample of said admixture, said method comprising the steps of:

(a) Contacting excess titanium hydride with said sample in the presence of aqueous sulfuric acid, said acid containing from about 33% to about 43% by Weight hydrogen sulfate, and thereafter removing substantially all undissolved solids from the resulting solution;

(b) Adding excess ammonium persulfate to the substantially-solid free solution thus formed to oxidize the tripositive titanium ions; and

(c) Titrating the chloride ion with a standard aqueous solution of silver nitrate. 10. The method according to claim 9 werein said aqueous sulfuric acid contains about 38% by weight hydrogen sulfate.

References Cited by the Examiner Mellors Comprehensive Treatise on Inorganic and Theoretical Chemistry, Supplement II, Part I (1956),

1Q Longmans, Green and Co., New York, pages 670-673.

REUBEN EPSTEIN, Primary Examiner. 

1. THE METHOD OF DETERMINING THE AMOUNT OF PERCHLORATE ION PRSENT IN A SAMPLE OF PERCHLORATE-CONTAINING MATERIAL, SAID METHOD COMPRISING THE STEPS OF: (A) REDUCING THE PERCHLORATE IONWITH EXCESS TITANIUM HYDRIDE IN THE PRESENCE OF AQUEOUS SULFURIC ACID; (B) OXIDIZING THE EXCESS TRIPOSITIVE TITANIUM ION WITH EXCESS AMMONIUM PERSULFATE; AND (C) THEREAFTER TITRATING THE CHLORIDE ION WITH A STANDARD AQUEOUS SOLUTION OF SILVER NITRATE. 